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Full text of "The manufacture and properties of iron and steel"

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-''•'•   HEAT  TRMTMElSrT.         • :                         '311
-carbon." JThe cause of evolution of heat at this point in cooling is'eonsldsered to-be the change from hardening to'cement "carbon, ••but <no satisfactory explanation is given by this theory for the changes • at the second and third critical points.
The allotropic theory holds that the iron of the steel is in different allotropic forms between the different critical points, and that below the second critical point the iron exists as alpha iron, but at this point beta iron is formed, and at th:e upper gamma, the .carbon being diffused in the iron. The cause of the evolution of heat is explained by the change from gamma to beta iron at Ar3, from beta to alp.ha at Ar2, while at Arx the carbon combines with alpha iron to form Fe3C, The retention of a hard allotropic state of iron, this retention being helped by the presence of carbon, is considered to be the cause of hardening.
The cartio-atto tropic theory is similar to the allotropic theory, except that hardening is supposed to be due to the retention by sudden cooling of a hard carbide of iron.         ,                       ;
The] Phase Doctrine. Prof.: Bakhuis-Eoozeboom explains* ttie detail, of.the Phase. Doctrine, a phase being denned as a mass chemically, or physically homogeneous, or as a mass of uniform concentration. Thus he states that a phase maybe liquid or solid, may be an jelement or a compound,! or a homogeneous mixture of variable concentration. Carbon, alpha, beta and gamma iron, liquid solutions, solid solutions of carbon in gamma iron or martensite, 'cementite and ferrite' are all phases, while pearlite is a conglomerate of phases. He gives a diagram shown in Pig. XV-H, which is intended to show the critical changes of alloys of iron and carbon containing different percentages of carbon at different temperatures.
Prom this it may be seen that the area, PSTJSI", represents the
Structure of slowly Cooled steels containing less than .89 per cent.
•of carbon, and SKLT the structure of high carbon steels cooleol
slowly.    MOSP is the region between At and A2, showing alph'a
'iron, while GOM is that between A2 and A3, beta iron.    Above G-OS,
-which is1 the line A3 in Pig. XV-A, the iron is in the phase gamma,
the micro-structure being 100% martensite.    As  shown by the
curve, .SB, the higher the carbon in the steel the higher the heat
needed to prevent the separation of cementite.    Thus m in a 1.00
C steel is the temperature necessary to hold in solution the excels
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* ZeitscHrift fur PhysikaUscne cnemie, Vol. XXXIV, 1900. 1. .and S. Inst, September, 1900.                                                                                     .                    '